The difference in pronunciation of "måltidet" between Norwegian and Danish is very striking! :)
Normally, for the plural, there is supposed to be an "r", here it there is not. Drengene is then an exception? otherwise it should have been "Drengerne". Is it for a purely phonetic reason or is there any rule about when not to put the "r"?
There are two was of forming the plural. One is with 'er' ones just an additional 'e'. But theres not really a rule when to use which. You just have to memorize it. So no its not an exeption, just one of two possibilities.